The present invention relates generally to magnetic random access memory (MRAM) devices, and more specifically, to the read direction for spin-torque based memory devices.
A spin torque magnetic random access memory (MRAM) device uses a spin-torque based memory element, for example, including a pinned layer, a tunnel barrier layer and a free layer in a magnetic tunnel junction (MTJ) stack. The magnetization of the pinned layer is fixed in a direction such that when current passes through the MTJ element, the free layer becomes either parallel or anti-parallel to the pinned layer. Resistance of the MTJ element depends on the relative orientation of the free layer and the pinned layer. When the free layer is parallel to the pinned layer, the MTJ element is in a low resistance state (e.g., a “0” memory state) and when they are anti-parallel, the MTJ element is in a high resistance state (e.g., a “1” memory state).
During the reading of data, a small current flows through the MTJ element and its resistance is compared with a pre-written MTJ cell (i.e., a reference cell), to determine whether the MTJ element being read is in a high or low resistance state. A problem associated with spin-torque memory devices is that the act of reading the MTJ element may disturb the data. In order to read the resistance of the MTJ element, current is passed through the MTJ element and the voltage across it is measured. This current may accidently write the MTJ element causing a “read” disturb.
During a read operation, even though a fixed voltage may be used to read, the voltage across the MTJ element during a read operation varies from MTJ element to MTJ element because the read voltage is placed across the MTJ element and transistor in series. Fluctuations in the resistance of the MTJ element and the resistance of the corresponding transistor may cause a fluctuation of read voltage on the MTJ element. Furthermore, this read voltage on the MTJ element depends on whether the MTJ element is in a high resistance state or a low resistance state during the read operation.